The present invention relates to a nitride-based semiconductor element such as a semiconductor laser, a light-emitting diode, an electronic device, or the like and a method for manufacturing the same.
Recently, attention is being drawn to nitride-based semiconductors such as, mainly, GaN, as the material for light-emitting diodes (LED) and semiconductor lasers (LD) which emit short-wave light ranging from the blue color to the ultraviolet region. As the substrates for growth of the above-mentioned nitride-based compound semiconductors, sapphire substrates are used most popularly, and, besides, SiC substrates, MgAl.sub.2 O.sub.4 substrates, etc. are also used.
As the method of forming a nitride-based compound semiconductor layer such as, mainly, GaN on a sapphire substrate or a SiC substrate, the metallo-organic chemical vapor deposition (MOCVD) method is used most popularly. If, in case a nitride-based compound semiconductor layer comprising GaN or the like on a sapphire substrate by the use of MOCVD method, the nitride-based compound semiconductor layer is grown directly on the substrate at a temperature (1000.degree. C. or higher) at which single crystal can grow, an island-shaped growth is markedly effected, and thus, no film with a good flatness can be obtained.
Thus, there is used the method according to which, before growing a nitride-based compound semiconductor layer at a temperature of 1000.degree. C. or higher at which generally the growth of single crystal is possible, a buffer layer composed of AlN or GaN is grown on the substrate at a low temperature of about 500 to 600.degree. C. By the insertion of the buffer layer thus formed, the growth of a nitride-based compound semiconductor layer having a good flatness has become possible.
However, at the early stage of growth at the high temperature, islands each growing around a growth nucleus are formed; and, when these islands coalesce, crystal defects such as dislocations, stacking faults, tubular holes, etc. are produced at a high density on the boundaries between the islands. In the case of the growth of a nitride-based compound semiconductor layer on a sapphire substrate, dislocations are produced at such a high density as about 10.sup.8 to 10.sup.11 cm.sup.-2, and these dislocations exist within the grown layers as threading dislocations which have propagated in the direction of growth. Therefore, these threading dislocations extend out to surface of the grown layers through the active layer in the element structure of a nitride-based short-wave semiconductor laser or the like.
Here, the dislocations induced when the islands coalesce in the case of an island-shaped growth will be described by reference to FIGS. 1A to 1C. As shown in FIG. 1A, growth nuclei are formed at random at the early stage of the growth. In this case, it is not possible to execute control over the positions at which the dislocations are produced and the positions at which such dislocations are not produced. As the growth goes on, threading dislocations are caused due to the coalescence of the islands as shown in FIG. 1B. The dislocation density can be controlled through the growth condition, but the limit of the control is to reduce the dislocation density as far as 1.times.10.sup.8 cm.sup.-2. Further, even if the thickness of the nitride-based compound semiconductor layer is increased as shown in FIG. 1C, the threading dislocation remains. In case elements have been fabricated, dislocations at the density of 10.sup.8 to 10.sup.11 cm.sup.-2 turn out to exist.
As the adverse effects exerted by the existence of dislocations on nitride-based semiconductor elements, the following ones are pointed out:
(1) A nitride-based compound semiconductor layer which has a high crystal defect density is poor in crystallizability, and the electrical characteristics are inferior. That is, the concentration of the background donor (carriers) is high, and the Hall mobility is small. PA1 (2) In the case of a nitride-based compound semiconductor layer having a high crystal defect density, the optical characteristics thereof are inferior in that, for instance, the lifetime of the carriers is short, and the luminous intensity is low. PA1 (3) At the tip end of a threading dislocation (the point at which the threading dislocation intersects the surface of the grown layer), pits (holes) are apt to be produced; for instance, in the process through which an InGaN-based active layer of the multi-quantum well structure (MQW) fabricated by growing a thin film of about, e.g. several nm, pits are formed, so that the orderliness of the multi-quantum well structure is disturbed, the desired structure in the luminous region of the element is spoiled, as a result of which the luminous pattern in the grown surface becomes non-uniform. PA1 (4) Due to the existence of the pits, the surface flatness (morphology) of the multi-layer film for an element such as a semiconductor laser or the like is spoiled, which results in the occurrence of inconveniences in the formation of the electrodes. PA1 (5) When electrical power is fed, the electrode material diffuse through the threading defects such as tubular holes etc. PA1 (6) When electrical power is fed or when a heat treatment is performed, a dopant such as Mg or the like diffuses into layers other than a desired layer into which a doping has been made intentionally, thus adversely affecting the element. PA1 a first layer, PA1 a mask formed on the first layer, the mask having a plurality of opening portions, PA1 a nitride-based compound semiconductor layer formed on the mask, the nitride-based compound semiconductor layer including PA1 a desired element structure formed on the semiconductor layer. PA1 a first layer, PA1 a mask formed on the first layer, the mask having a plurality of opening portions, PA1 a nitride-based compound semiconductor layer formed on the mask, the nitride-based compound semiconductor layer including PA1 a first region which has threading dislocations produced in such a manner that, in approximately a middle portion between two adjacent opening portions of the plurality of opening portions in the mask, a plurality of dislocations extend vertically to a surface of the mask, and PA1 a second region which comprises portions other than the middle portions and free from the dislocations, and PA1 a light-emitting layer formed substantially on the second region of the nitride-based compound semiconductor layer, the light-emitting layer being sandwiched between current injections layers. PA1 the nitride-based compound semiconductor layer comprises: PA1 the first layer has a plurality of projections periodically formed on a surface thereof, PA1 on surfaces of the projections, the un-opened portions of the mask are formed, and PA1 in the opening portions of the mask, the underlying layer forms a junction with the nitridebased compound semiconductor layer. PA1 the step of forming, on a first layer, a mask which has a plurality of opening portions, PA1 the step of depositing a nitride-based compound semiconductor layer, on the first layer on which the mask has been formed, by utilizing lateral growth on the mask in such a manner that a region in which crystal defects propagating in a direction of perpendicular to a surface of the mask are small in number is formed, and PA1 the step of forming a desired element structure on the nitride-based compound semiconductor layer.
Due to inconveniences as mentioned above, the initial characteristics and reliability of the element are spoiled. However, the problem concerning the reduction in the density of crystal defects such as threading defects etc. cannot be solved by the conventional methods within the range of the optimization of the growth condition of the low-temperature buffer layer and the growth condition, at a high temperature, of nitride-based compound semiconductor layer, and other measures.
Further, among the nitride-based compound semiconductors, particularly the InGaAlN series are hopeful for use as short-wave light sources containing the ultraviolet rays, thus attracting attention. However, in the case of this series of material, the low-refraction layer containing aluminum cannot be grown thick, so that no sufficient optical confinement can be effected. Due to this, there arises problems such as the problem that the threshold value is high and the mode is not stabilized.
As stated above, in the case of a light-emitting element and an electronic device fabricated by the use of a nitride-based semiconductor layer, crystal defects such as threading dislocations etc. are produced at a high density even if growth techniques for the growth of, e.g. low-temperature growth buffer layers are used, and, due to the existence of these crystal defects, the initial characteristics of and reliability for the elements are kept from being improved.